Introduction—Inheriting 800 Years of Craftsmanship Through Digital Technology

The Japanese sword is not merely a weapon. It is the crystallization of over 800 years of sword-forging technique, a work of art in which the craftsman's soul dwells in the microscopic details—the hamon (blade pattern), the jihada (grain structure), the nakago (tang)—that are easily overlooked by the naked eye. Yet even swords designated as National Treasures or Important Cultural Properties face constant risk of loss due to fire, earthquakes, and deterioration over time.

Against this backdrop, efforts to record and preserve Japanese swords as precise digital data have accelerated rapidly in recent years. Museums, research institutions, and private enterprises collaborate to build "digital archives" using CT (computed tomography) scanning and high-precision 3D scanners. This article examines the frontlines of these efforts from three perspectives: technology, case studies, and challenges.

Recording Sword Blades with CT Scanning and 3D Scanners

CT scanning is a groundbreaking technology that enables non-destructive analysis of blade structure. It can three-dimensionally visualize the layered structure of tamahagane, the junction between shin'gane (core steel) and kawagane (outer steel), and the steel distribution created by repeated folding and forging.

The Tokyo National Museum has conducted micro-CT imaging of swords designated as Important Cultural Properties, achieving internal structural analysis with resolution below 0.1mm. This makes it possible to confirm forging methods difficult to distinguish from external appearance alone—such as the presence or absence of hon'sanmai (true three-layer construction), koubuse (outer-layer wrapping), or muneyaki (back edge hardening)—and to read the history of past repairs and polishing. The ability to "see" the interior without grinding down the blade surface is extremely significant from a cultural property protection standpoint.

Conversely, external measurement using 3D scanners can record the curvature (sori), blade length (hachō), and changes in width of the base and point with 0.05mm precision. Compared to conventional hand measurement, reproducibility is high, and tracking long-term changes over decades becomes much easier. For sword researchers, it has become a powerful tool for quantitatively comparing the "appearance" (sugata) of individual blades.

Technical Challenges in Digital Preservation of Hamon

One of the greatest beauties of the Japanese sword is the hamon. This is the white pattern created during the hardening of the blade edge, and it varies in design according to school and swordsmith—including chōji (clove bud), guno-me (interlocking circles), and suguha (straight pattern). However, digital recording of hamon is extremely challenging technically.

The reason is that hamon appearance changes dramatically depending on the angle of illumination. Under light sources from different angles, it presents an entirely different appearance, making it impossible for a single photograph to fully capture its original beauty. To address this problem, growing attention is being paid to RTI (Reflectance Transformation Imaging) technology. Multiple images captured under illumination from various angles are composited to recreate surface conditions at any desired light source direction after photography.

In the "Sword Cultural Property Database" project, carried out in collaboration between the Agency for Cultural Affairs and the NPO Japanese Institute for Conservation of Cultural Property, detailed hamon recording through RTI imaging is being attempted. As objective hamon data accumulates independent of display lighting conditions, comparative analysis among researchers becomes exponentially easier.

Furthermore, "polarized 3D scanning" technology for high-precision measurement of glossy metal surfaces is approaching practical implementation. While conventional structured-light scanners are hindered by mirror reflection, combining polarizing filters makes it possible to capture even fine surface irregularities of the blade. Application to recording the grain of jihada—such as itame (wood grain), mokume (burl), and masame (straight grain)—is also anticipated.

Real Examples of Online Museums and Virtual Exhibitions

The results of digital archiving are advancing toward public release in the form of virtual exhibitions.

At the Tokyo National Museum's "TNM & TOPPAN Museum Theater," virtual sword experiences using VR have been implemented. The museum's process involves 3D scanning of a National Treasure tachi (long sword) and producing it as life-size VR content for observation. The ability to freely observe from all directions a blade that in physical exhibition could only be viewed at limited angles through display cases has been highly praised.

Kyoto National Museum participates in the "e-Kokuho" project and releases image data of its holdings of Important Cultural Properties and National Treasures. For swords, this includes close-up images of the nakago (tang) and mei (signature), as well as photographs of the hamon taken immediately after polishing, providing high value as research materials.

In the private sector, the sword specialist portal "Touken World" provides free public access to a database of over thousands of sword images. In addition to enabling cross-search of holdings across the nation's art museums and museums, it also offers interactive content explaining each part of the sword using 3D models, serving as introductory material for beginners.

Internationally, the Metropolitan Museum of Art in New York, under its "Open Access" policy, releases metadata and high-resolution images of its Japanese sword collection under CC0 license. An environment where researchers can freely download and use such materials has dramatically promoted international sword research.

Current State and Challenges in Digital Technology Utilization

The adoption of digital archives is proceeding steadily, but many challenges remain to be addressed.

Lack of Data Standardization: Each institution records swords using independent shooting conditions and data formats, making cross-comparative analysis difficult. International standards for unified digital data formats of swords remain undeveloped, and infrastructure development for integrating and utilizing data from different institutions is urgently needed.

Shortage of Specialized Personnel: Specialists possessing both expertise in 3D scanning and image processing and knowledge of sword appraisal are extremely rare. Both technical skill and discerning eye are essential, from setting shooting conditions to interpreting acquired data, and cultivating such personnel requires considerable time and systematic education.

Cost Barriers: High-precision 3D scanners require equipment investments ranging from millions to tens of millions of yen. The barriers to adoption are high for regional art museums and individual collectors, with a tendency for digitization to concentrate in large public institutions. As of 2026, initiatives by small and medium-scale institutions using crowdfunding and subsidies have begun in some cases.

Expectations and Limitations of AI Appraisal: Research using machine learning to infer creator and school from hamon patterns is progressing. While certain accuracy has been reported in blade pattern classification experiments using deep learning models, appraisal requires multifaceted elements including naked-eye observation, tactile sensation, and weight balance, and AI has not reached the stage of replacing expert experience. Practical application remains as a "supplementary tool."

Future Prospects—The Fusion of Digital Technology and Tradition

Digital archiving brings new possibilities to the "transmission" of Japanese swords.

First, reduction of loss risk. Loss through fire, earthquake, and theft represents the greatest threat to cultural property, but precise digital data serves as a key to restoration. The case of 3D scan data being utilized in restoration deliberations following the 2019 Shuri Castle fire raised society's awareness of the importance of digital preservation of cultural property. This lesson is being applied to the sword field as well, accelerating prioritized digitization at major holding institutions.

Second, transmission of swordsmith technique. By precisely analyzing the works of retiring swordsmiths, it becomes possible to quantify the characteristics of forging methods and build a "digital master" that next-generation craftspeople can reference. There is potential to visualize and share, in data form, some of the techniques that have been transmitted through oral tradition and imitation.

Furthermore, promotion of global sword research. Comparative research with Japanese swords in foreign collections becomes exponentially easier through digital data sharing. Particularly in provenance investigations of swords lost after World War II, the role played by digital data is significant, serving as a key to promoting international cooperation among holding institutions.

Digital archiving of the Japanese sword extends far beyond mere "preservation," holding the potential to revolutionize all phases of research, education, appreciation, and transmission. From the perspective of fusion of tradition and cutting-edge technology, the sword field's efforts are becoming a model case for other cultural property domains.